The origins of the Spanish influenza are still a matter of conjecture. Before and after 1918, most influenza pandemics begin in Asia and spread outward to the rest of the world. In 1918, however, influenza seemed to arise in three separate locations—in Europe, Asia, and North America—simultaneously.

In the spring of 1918, the first wave of influenza hit, followed by a much deadlier second wave in the fall of 1918 and a third wave in early 1919. The first wave was largely unnoticed; flu was not a reportable disease to the public health authorities and the early flu cases were mostly mild. Pathologists did note in cases of fatalities that victims had lungs full of liquid and showed signs of hemorrhaging. Young people, usually rather impervious to the flu, also seemed strangely susceptible to form of influenza.

Military camps and installations in Europe and the United States, overcrowded with young people, were particularly hard hit. The earliest recorded outbreak may have come from Haskell, Kansas. A local doctor, Loring Minor, noted cases of influenza in January and February 1918 that seemed to affect the strongest and healthiest people in the area. From here it may have travelled via military recruits to Camp Funston, Kansas, where an epidemic of the flu began in early March. The first official recorded case of the Spanish influenza was one Private Albert Gitchell, mess cook, who reported to the infirmary with chills, fever, aches, and a sore throat. By the end of the week there were 522 more cases at the camp.

Because of troop movements between bases, flu spread to other military populations in the United States and eventually found its way over to Europe aboard troop ships from America. It spread rapidly throughout war-torn Europe, affecting troop morale and disrupting wartime operations. From Europe, the flu spread to Africa, Asia, and the Pacific. The spring wave waned during the summer months, although individual pockets of a much more lethal kind of influenza popped up in isolated locations.

In the fall, a newer and more deadly second wave of influenza arose. It is likely that this was a mutated form of the earlier, milder version of the spring influenza, and the first reports came from Brest, a seaport in France. Almost simultaneously, the port cities of Boston, Massachusetts and Freetown, Sierra Leone experienced outbreaks of the virus. Cities and towns on the coasts of countries began to see severe outbreaks, as shipping and troop movement abetted the transmission of the disease. By November, 1918, almost no corner of the world was free from the disease.